In scanning microscopy, a sample is illuminated with a light beam in order to observe the detection light emitted, as reflected or fluorescent light, from the sample. The focus of an illumination light beam is moved in a specimen plane by means of a controllable beam deflection device, generally by tilting two mirrors, the deflection axes usually being perpendicular to one another so that one mirror deflects in the X direction and the other in the Y direction. Tilting of the mirrors is brought about, for example, by means of galvanometer positioning elements. The power level of the light coming from the specimen is measured as a function of the position of the scanning beam. The positioning elements are usually equipped with sensors to ascertain the present mirror position.
In confocal scanning microscopy specifically, a specimen is scanned in three dimensions with the focus of a light beam.
A confocal scanning microscope generally comprises a light source, a focusing optical system with which the light of the source is focused onto an aperture (called the “illumination pinhole”), a beam splitter, a beam deflection device for beam control, a microscope optical system, a detection pinhole, and the detectors for detecting the detected or fluorescent light. The illumination light is coupled in via a beam splitter. The fluorescent or reflected light coming from the specimen travels back through the beam deflection device to the beam splitter, passes through it, and is then focused onto the detection pinhole behind which the detectors are located. Detection light that does not derive directly from the focus region takes a different light path and does not pass through the detection pinhole, so that a point datum is obtained which results, by sequential scanning of the specimen, in a three-dimensional image. A three-dimensional image is usually achieved by acquiring image data in layers, the path of the scanning light beam on or in the specimen ideally describing a meander (scanning one line in the X direction at a constant Y position, then stopping the X scan and slewing by Y displacement to the next line to be scanned, then scanning that line in the negative X direction at constant Y position, etc.). To make possible acquisition of image data in layers, the sample stage or the objective is shifted after a layer is scanned, and the next layer to be scanned is thus brought into the focal plane of the objective.
German Unexamined Application DE 199 06 757 discloses an optical arrangement in the beam path of a light source suitable for fluorescence excitation, preferably in the beam path of a confocal laser scanning microscope, having at least one spectrally selective element for coupling the illumination light of at least one light source into the microscope and for blocking out the illumination light scattered and reflected from the specimen, or the excitation wavelength of the light coming out of from the specimen via the detection beam path. For variable configuration with a very simple design, the optical arrangement is characterized in that illumination light of differing wavelengths can be blocked out by way of the spectrally selective element. Alternatively, an optical arrangement of this kind is characterized in that the spectrally selective element can be set to the excitation wavelength that is to be blocked out. The spectrally selective element can be embodied as an acoustooptical detector (AOD) or an acoustooptical tunable filter (AOTF). In a preferred embodiment, a scanning microscope that utilizes and detects the dispersive properties of the spectrally selective element is disclosed.
The optical arrangement known from the existing art has the disadvantage that only illumination light which has narrow spectral widths, i.e. a few nanometers, can be blocked out of the light coming from the specimen or sample. This is particularly disadvantageous when spectrally broad-band light sources are used.